This disclosure relates to a centrifugal compressor. More particularly, the disclosure relates to such a compressor having a diffuser with microjet fluid injection ports for increased stable operating range.
The main energy conversion elements of a centrifugal compressor are its impeller and diffuser. Inside the rotating impeller of the centrifugal compressor mechanical shaft energy is transferred into fluid energy. The fluid leaving the impeller has increased pressure and increased velocity. Roughly only half the energy transfer accomplished by the impeller is in the form of increased pressure (potential energy) while the remainder is in the form of high fluid velocity (kinetic energy). Since the purpose of a compressor is to increase pressure, centrifugal compressors are equipped with a diffusing element downstream of the impeller. In the diffuser high velocity kinetic energy is converted into potential energy increasing the overall pressure rise and therefore the overall efficiency of the compressor. The diffuser element downstream of the impeller can be either vaneless or vaned, depending on the specific requirements of the compressor.
At low-capacity off-design operating conditions the diffuser will be the element of the centrifugal compressor that causes flow instability preventing stable compressor operation. To increase the stable operating range of a centrifugal compressor at low flow conditions, variable geometry inlet guide vanes as well as variable geometry vaneless and vaned diffusers have been introduced. Variable geometry inlet guide vanes increase the stable operating range of a centrifugal compressor at lower flow rates through the increased throttling action at more closed guide vane positions. Variable geometry diffusers adjust the diffuser cross-sectional flow area to the low flow rate encountered under part-load conditions, thus maintaining flow angles and velocities similar to those at full-load design conditions.
Part-load stability has also been accomplished by recirculating discharge flow through the diffuser of the compressor. Instead of reducing the size of the diffuser to match the lower flow rate at part-load, increased stable operating range obtained by increasing the flow in the diffuser to full-load values through flow recirculation, again maintaining flow angles and velocities similar to those at full-load design conditions.
However, all variable geometry and recirculating flow concepts increase compressor cost and complexity considerably. These concepts are known to cause reliability issues and reduce centrifugal compressor efficiency at both design (due to increased leakages) and off-design conditions (due to full-load frictional losses at part-load flow conditions) considerably.